Wideband communication systems operating in the gigahertz region are considered to be an applicable environment for optical fibers and, of course, light sources. At these high frequencies, it is important for the fibers and sources to be compatible in order to reduce dispersion and coupling losses. Compatibility is improved and fiber dispersion loss is reduced by operating each light source in a single mode to produce a narrow spectral output, that is, an output substantially at a single wavelength. Single mode is generally understood to mean single longitudinal mode.
In order to achieve single longitudinal mode behavior for light sources and, particularly, for lasers, at least one important type of laser has been developed, namely, the coupled-cavity semiconductor laser. The coupled-cavity semiconductor laser comprises multi-section (two or more sections) coupled-cavity structures which are realized as several coupled active semiconductor cavity sections. See, for example, L. A. Coldren et al., Appl. Phys. Lett., Vol. 38, pp. 315-317 (1981); L. A. Coldren et al., IEEE J. of Quantum Electronics, Vol. QE-18, No. 10, pp. 1679-1688 (1982); K. Ebeling et al., Elect. Lett., Vol. 18, No. 21, pp. 901-902 (1982); M. B. Chang et al., IEEE J. of Quantum Electronics, Vol. QE-16, No. 9, pp. 997-1001 (1980); and U.S. Pat. No. 4,284,963 issued to L. Allen, Jr. et al. (August 1981).
For single longitudinal mode operation to occur, the individual cavity sections must be tuned to a selected longitudinal mode of the coupled-cavity laser. However, since the wavelengths of the longitudinal modes are affected by temperature and modulation bias current as well as by aging of the semiconductor material, the single longitudinal mode condition is not likely to be stable over a range of modulation or temperature conditions. As a result of this instability, multiple longitudinal modes are expected to appear in the light output from the coupled-cavity laser. Hence, it is important to control the coupled-cavity laser so as to provide substantially single longitudinal mode operation over extended periods of time despite the effects of modulation or temperature variations or the like.